Not applicable.
This invention pertains generally to a surface or xe2x80x9ccappingxe2x80x9d layer for optics used for lithographic applications, and particularly for extreme ultraviolet lithography. The surface or xe2x80x9ccappingxe2x80x9d layer disposed on an optic, such as a multilayer mirror, acts in combination with molecular species such as O2, H2 and H2O in the presence of energetic radiation to maintain the optic surface substantially free of carbon contamination. The capping layer is such that it also protects the surface from oxidation or oxidative degradation.
The demand for circuitry of increasing density and higher resolution features and thus, for smaller feature sizes has inexorably driven the wavelength of radiation needed to produce the desired pattern to ever-shorter wavelengths. Extreme ultraviolet (EUV) radiation, i.e., radiation in the wavelength range of 3-15 nm, is strongly attenuated by conventional transmission optics and thus lithographic projection methods that use optics that reflect rather than transmit light become necessary. At the present time, the reflective optics and masks used for extreme ultraviolet lithography (EUVL) are based on a Mo/Si multi-layer structure with the topmost layer consisting of about 40 xc3x85 of Si.
Hydrocarbon molecules and water vapor are ubiquitous in the photolithographic apparatus (stepper) used for EUVL. They are present in the background environment of the stepper and can arise from oils used in vacuum pumps that maintain the required low pressure atmosphere within the stepper as well as other organic materials within the stepper environment such as photoresist materials and cabling. Water is adsorbed on all surfaces and is very difficult to remove in spite of the use of rigorous vacuum baking techniques. EUV radiation is energetic enough to cause the decomposition of water molecules adsorbed on or proximate to a surface to produce hydrogen and reactive oxygen species that can attack, degrade, or otherwise contaminate optical surfaces. EUV-induced dissociation of organic materials can cause the surfaces of multi-layer Mo/Si optics and masks to become contaminated with carbon. These carbon coatings gradually degrade the performance of the coated component, e.g., reduce the reflectivity of optical surfaces. This phenomenon is shown graphically by curve 100 in FIG. 1 where relative reflectivity (R/R0) is plotted against exposure time for a multilayer Mo/Si mirror exposed to a hydrocarbon vapor (neoprene) in a flux of EUV radiation. It can be seen that after 3 hrs. exposure there is an unacceptable 7% loss in reflectivity.
A method that can be used for removing carbon contamination is the use of oxygen-containing molecules, such as H2O and O2 which in the presence of energetic radiation can form reactive chemical species that can react with carbon contamination to form volatile carbon compounds, such as CO and CO2, that can be pumped away. While the oxidative treatment works to remove carbon contamination, once the carbon layer is removed from the surface of the Mo/Si multi-layer optic structure, the reactive oxygen species can oxidize the underlying terminating Si surface layer, forming silicon oxides (generally SiO2). Oxygen is a strong absorber of EUV thus, oxidation of the Si-terminated Mo/Si optic would cause irreversible degradation of the reflective ability of these optics, such as that shown in FIG. 1.
Recognizing the degradation in performance of optical components associated with interactions with environmental contaminants, various solutions have been proposed. By way of example, gas plasmas have been found to be effective for removing surface contaminants, and in particular carbon contamination, such as disclosed in U.S. Pat. No. 5,814,156 to Elliot, U.S. Pat. No. 5,312,519 to Sakai et al., and by Mxc3xcller et al., Rev. Sci. Instrum., 63, 1428-1431, 1992. However, being unable to precisely control the extent of reaction, unwanted and harmful reactions with system components, as discussed above, can take place.
U.S. Pat. No. 5,958,605 to Montcalm et al. discloses an overcoat (or xe2x80x9ccappingxe2x80x9d) layer applied to the surface of a Mo/Si multilayer mirror structure that resists oxidation. In order that the reflectivity of the multilayer optic is not degraded by the overcoat layer, it is necessary to tailor both the thickness of the overcoat layer and the reflecting layer that lies directly beneath it such that the beams reflected from these layers are in phase and add constructively. This can require a complex series of calculations to optimize the thickness of either the overcoat layer, the thickness of the layer beneath the overcoat, or both. Moreover, the overcoat layer of Montcalm does not act to prevent carbon contamination of the multilayer optic.
The requirements for components for EUV lithography are such that any degradation in reflectivity of multilayer optics, such as would be caused by the formation of a carbon film, is unacceptable. Present methods for removing carbon by oxidation are difficult to apply in the environment of an EUV lithography system and are often harmful to optical components. Oxidation barriers, such as described above, are difficult to use because of reflectivity considerations. An oxidation resistant layer that could be disposed on top of a terminal Si layer in an multilayer Mo/Si mirror structure, that would transmit substantially all the incident EUV radiation, and would actively prevent or minimize carbon contamination of the multilayer Mo/Si mirror surface would provide the best approach for solving this problem.
The present invention is directed generally to a self-cleaning optic, whereby a metal surface or xe2x80x9ccappingxe2x80x9d layer, in combination with gaseous reactants such as O2, H2, H2O and combinations thereof, provides for continuous cleaning of carbon deposits from the surface of the multilayer reflective optics used for lithographic applications and particularly for extreme ultraviolet lithography (EUVL). The gaseous species can be adventitious in the environment of the optic or added deliberately. The metal used for the capping layer is required to have the ability to transmit substantially all ( greater than 90%) of the incident EUV radiation to the underlying multilayer mirror structure, be resistant to oxidation, particularly in the presence of EUV radiation, and provide for catalytic dissociation of the gaseous reactants. The invention is further directed to methods of making the self-cleaning optic.
Currently, the reflective optics and masks used for EUVL are based on a Mo/Si multi-layer structure with the topmost layer consisting of about 40 xc3x85 of Si. In this invention, a thin (≈5-400 xc3x85 and preferably 5-10 xc3x85) metal capping layer is deposited on top of the terminating Si layer. In some cases, it can be desirable to interpose a binding layer between the terminating Si layer and the capping layer to promote adhesion of the capping layer. However, it is necessary that this binding layer be thin (≈1-3 xc3x85) so as not to affect materially transmission of incident radiation. A small pressure (≈10xe2x88x924-10xe2x88x928 Torr) of one or more of the aforementioned reactant gas species in the environment of the capped multilayer optic, and irradiated by EUV, will generate reactive atomic species which, in turn, will react with and remove any carbon contaminants deposited on the optic surface.
In addition to oxidizing species that can be produced by the decomposition of O2 and H2O by EUV radiation, atomic hydrogen can also be used to remove carbon from surfaces as CH4, for example. Thus, it is desirable that the metal comprising the capping layer provide a catalytic surface for the binding and dissociation of molecular hydrogen to form atomic hydrogen. Metals that are suitable for the capping layer include Ru, Rh, Pd, Ir, Pt, and Au and combinations thereof.